A printhead assembly of an ink jet printer typically includes one or more printheads each having a plurality of ink jets from which drops of ink are ejected towards an image receiving surface, such as a media sheet or intermediate transfer surface. During operation, drop ejecting signals activate actuators in the ink jets to expel drops of fluid from the ink jet nozzles onto the image receiving surface. By selectively activating the actuators of the ink jets to eject drops as the image receiving surface and/or printhead assembly are moved relative to each other, the deposited drops can be precisely patterned to form particular text and graphic images on the recording medium.
As is known in the art, different printheads can have various drop position differences and these can modify the intended output of an image and ultimately results in image artifacts such as banding or different levels of graininess and/or clustering. This can be true even if the resolution and drop mass generated by the printheads are the same. Such differences may be introduced from part or electronic tolerances, etc., for example, during manufacture and assembly of the printheads. There are a number of important drop position responses of a printhead which are routinely performed during manufacture and/or calibration. For example, drop position can be adjusted by modifying the driving signals to the actuators of the ink jets as well as the operating temperatures of the printheads. These adjustments have traditionally been sufficient to satisfy customer needs. This is particularly true in an ink jet printer that utilize a single printhead.
Drop position differences are more of an issue when two or more printheads are arranged side by side in an imaging device. Differences in the graininess of images produced by printheads arranged side by side in a printer can result in more severe visually noticeable and objectionable image quality defects, such as streaking and banding that extend in the process direction of a printed image. This is true during the initial manufacture of a device, as well as maintenance and calibration needs as a device ages. As mentioned, the graininess and/or clustering characteristics of images produced by a printhead may be adjusted for each printhead. In imaging devices that are configured to form images onto an intermediate transfer surface, e.g., a rotating drum or belt, prior to transfixing the image onto a media sheet, drop position differences between printheads may be detected by scanning the images on the drum using an image sensor and correlating the scans to a graininess level for the printheads in a known manner. Once a graininess level has been determined for the printheads, the graininess level for one or more of the printheads can be adjusted in an effort to normalize the printheads so that the images produced by adjacent printheads have approximately the same level of graininess.
One difficulty faced in the graininess normalization routine described above is that the structure of images on the intermediate transfer surface is not easy to correlate to the graininess in an image. It is particularly difficult to measure and modify a single jet parameter to control the overall graininess in a half-toned image which is composed of numerous jets. What is needed is a specific pattern which can be easily measured on a single jet basis and corrected such that the overall graininess of the final image is improved.